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Top Forums UNIX for Beginners Questions & Answers One parent, multiple children pipe with fork() Post 303017885 by jim mcnamara on Wednesday 23rd of May 2018 07:58:15 PM
Old 05-23-2018
Code:
 else if(fork()==0)

You have three of these. No. any call to fork() creates a pid, you just lost the pid of the new process.

Code:
else  
{ mypid1 = fork();
  if (mypid!=0 ) 
  {
    // do something 
  }
}

And I don't see what your program logic should be using your current model. Get it working for one child process only. Set up pipes for that. As you have your code now there can be a race condition on pipes it seems to me.

If you have to have multiple children learn about IPC here, use a mutex on shared memory if you have to have this odd cascade of processes.

IPC: https://www.tldp.org/LDP/tlk/ipc/ipc.html
 

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fork(2) 							System Calls Manual							   fork(2)

Name
       fork - create a new process

Syntax
       #include <sys/types.h>
       #include <unistd.h>

       pid = fork()
       pid_t pid;

Description
       The  system  call causes creation of a new process.  The new process (child process) is an exact copy of the calling process except for the
       following:

       o    The child process has a unique process ID.

       o    The child process has a different parent process ID (that is, the process ID of the parent process).

       o    The child process has its own copy of the parent's descriptors.  These descriptors reference the same underlying objects, so that, for
	    instance,  file  pointers  in file objects are shared between the child and the parent, so that a on a descriptor in the child process
	    can affect a subsequent read or write by the parent.  This descriptor copying is also used by the shell to	establish  standard  input
	    and output for newly created processes as well as to set up pipes.

       o    The child processes resource utilizations are set to 0.  For further information, see

Return Values
       Upon  successful  completion,  returns  a  value  of  0	to the child process and returns the process ID of the child process to the parent
       process.  Otherwise, a value of -1 is returned to the parent process, no child process is created, and the global variable errno is set	to
       indicate the error.

Diagnostics
       The system call fails and no child process are created under the following conditions:

       [EAGAIN]       The system-imposed limit {PROC_MAX} on the total number of processes under execution would be exceeded.

       [EAGAIN]       The system-imposed limit {CHILD_MAX} on the total number of processes under execution by a single user would be exceeded.

       [ENOMEM]       There is insufficient swap space for the new process.

See Also
       execve(2), wait(2)

																	   fork(2)
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